Fix potential exception in KeepOldest split brain strategy (akkadotne…

…t#3272)

If remaining and unreachable lists are empty an exception will be
encountered as soon as .First() is executed to try to find the oldest
node

src/core/Akka.Cluster/SplitBrainResolver.cs

@@ -171,18 +171,23 @@ public IEnumerable<Member> Apply(NetworkPartitionContext context)
             var remaining = MembersWithRole(context.Remaining);
             var unreachable = MembersWithRole(context.Unreachable);
 
+            if (remaining.IsEmpty && unreachable.IsEmpty) // prevent exception due to both lists being empty
+            {
+                return new Member[0];
+            }
+
             var oldest = remaining.Union(unreachable).ToImmutableSortedSet(Member.AgeOrdering).First();
             if (remaining.Contains(oldest))
             {
                 return DownIfAlone && context.Remaining.Count == 1 // oldest is current node, and it's alone
                     ? context.Remaining 
                     : context.Unreachable;
             }
-            else if (DownIfAlone && context.Unreachable.Count == 1) // oldest is unreachable, but it's alone
+            if (DownIfAlone && context.Unreachable.Count == 1) // oldest is unreachable, but it's alone
             {
                 return context.Unreachable;
             }
-            else return context.Remaining;
+            return context.Remaining;
         }
 
         private ImmutableSortedSet<Member> MembersWithRole(ImmutableSortedSet<Member> members) => string.IsNullOrEmpty(Role)